This invention relates to apparatus for feeding ink ribbons and in particular for feeding thermal transfer ink ribbons and controlling the winding of used ribbon onto a take-up spool.
Ink ribbons for printing on a print receiving medium are commonly fed from a supply spool past a printing head where printing is effected and then the used ribbon is wound onto a take-up spool. Thermal transfer ink ribbons are usually maintained stationary relative to the print receiving medium and relative movement is effected between a thermal print head and the combination of thermal ink transfer ribbon and the print receiving medium. The thermal print head may be stationary and the print receiving medium together with the ribbon is fed past the print head. The thermal transfer ink ribbon extends between the elements of the print head and the print receiving medium. Printing is effected by selectively heating thermal printing elements arranged in a row on the print head during movement of the ribbon and medium past the elements. Heating of an element results in melting of the ink layer in the vicinity of the heated element and the melted ink is deposited on the surface of the ink receiving medium. Successive selective heating of the elements effects line by line printing of dots in selected positions to build up complete characters or patterns. Transfer of ink from the ribbon requires that the print receiving medium is urged into intimate contact with an ink layer of the ribbon. This is accomplished by means of a pressure roller which resiliently urges the print receiving medium into contact with the ribbon and the back of the ribbon into heat transfer engagement with the printing elements of the print head. When the print receiving medium is fed past the print head, the intimate contact between the ribbon and the print receiving medium ensures that sufficient frictional force is applied by the print receiving medium on the ribbon to feed the ribbon at the same speed as the feeding of the medium. Accordingly no additional feeding means are required to feed the ribbon with the medium.
After passage of the ribbon past the thermal print head, the used ribbon is peeled from the surface of the print receiving medium to leave the deposited ink pattern on the medium. The used ribbon tends to adhere to the print receiving medium and accordingly it is necessary to apply a small tension force to the used ribbon in a direction at an angle to the surface of the medium. Furthermore it is convenient to wind the used ribbon onto a take up spool to retain the used ribbon within the machine until subsequent disposal of the used ribbon. The tension required to peel the ribbon from the medium may be applied to the ribbon by rotationally driving the take up spool to wind the ribbon thereon. This is achieved by driving the take up spool such that it tends to wind the ribbon faster than the ribbon issues from the print head. The drive tot he take up spool may be through a slipping clutch or the drive motor may be stalled when the ribbon is under tension.
It is proposed to use thermal transfer printing in franking machines for printing franking impressions on mail items. The mail item is fed into the machine and is pressed into contact by means of a pressure roller with a thermal transfer ribbon in the vicinity of a thermal print head. As the mail item is fed past the print head, together with the ribbon, the franking impression is built up line by line by rows of printed dots. Upon completion of printing of the franking impression, the pressure roller is retracted to release the mail item to permit the item to be ejected from the machine. When the pressure roller is retracted to release the mail item, the mail item no longer imposes any restraint upon travel of the ribbon and as a result the tension applied to the ribbon to peel it from the mail item during printing of a franking impression tends to draw additional ribbon from the supply spool and wastage of ribbon occurs between consecutive franking operations.